The electrophotographic toner is conventionally composed of a resin composition, coloring agent which contains pigments or dyestuffs, control agents, wax and the like. Synthesic or natural resin is used for the resin composition singly or as a suitable mixture. A relatively high molecular weight styrene containing polymer is generally used as admixture with a relatively low molecular weight styrene containing polymer in a suitable proportion. The composition and thermal properties of the styrene containing polymer employed are important factors deciding the toner characteristics, and currently their improvement is strongly required.
Many kinds of fixation method which called hot-roll process are employed in the electrophotographic copying machines and printers. In this method, toner particles which were electrostatically transferred on copying papers are passed through heated press rolls, thereby the particles are melted and fixed on the paper. Copying machines have recently been developed for conducting duplication at a high rate with a low energy fixation. Conventional toner, however, is not always satisfactory for the performance of these machines and printers by the following reason. The heat quantity transferred from the hot rolls to the toner particles at high duplication speed is less than at low duplication speed. A remarkable decrease in the surface temperature of hot rolls is also caused by the increase in heat removal to the copying papers, which leads to insufficient fusion and deteriorated fixation of the toner particles. Besides the copying machines fitted with various auxiliary devices are required to operate all of these devices within a limited consumption of electricity. Since the proportion of electrical consumption for heating the rolls is very high in the total consumption, it is strongly required to lower the temperature of hot rolls. The conventional toner, however, cannot perform satisfactory melting and adhesion to the copying papers at lower surface temperatures of the hot rolls. Therefore it has been strongly desired to develop the toner which exhibits sufficient fixing ability with by supplying a smaller quantity of heat. As a countermeasure to this problem, there is a method for satisfactorily conducting the fixation under conditions of high speed and lower temperature conditions by lowering the softening temperature or melt viscosity of the resin constituting the toner particles. That is, the toner melting at the lower temperature can be prepared by adjusting the mixing ratio of the resin. This method, however, increases the proportion of low molecular weight styrene containing resin and drastically decreases the melt viscosity of resin at high temperatures. As a result, the toner particles which were melted and pressed on a copying paper by the hot rolls at the fixation stage are partly are transferred and left on the hot roll surface when the paper was removed from the rolls. The attached toner on the roll surface is pressed again on the next paper, which is so-called "offset phenomenon". The offset phenomenon causes a serious disadvantage that the copying papers are contaminated and good images cannot be obtained.
Besides the resin having a low softening temperature can be prepared by controlling the monomer composition which constitutes the resin. The toner derived from such low temperature softening resin, however, causes mutual adhesion of the toner particles by pressure during the storage. In the extreme cases, the toner is liable to occur so-called "blocking" phenomenon which leads to whole coagulation of the toner. Particularly in the case of using the low molecular weight styrene containing resin as a low temperature softening component, blocking resistance drastically deteriorates and causes severe problems in actual application.
The resin which is free from the blocking phenomenon and moreover has a low softening temperature can be prepared by similarly reducing molecular weight of the low molecular weight styrene containing polymer which constitutes the resin. Relatively large amount of a polymerization initiator, however, is uneconomically required in order to obtain such low molecular weight styrene containing polymer. Furthermore, a large amount of relatively high-polar impurities such as residue of the polymerization initiator is supposed to contaminate the toner. Thus frictional charge of the toner is extremely labilized and it becomes difficult to obtain sharp, clear and good images over a long period.
Besides the low molecular weight styrene containing polymer can also be obtained by using a small amount of polymerization initiator in combination with a chain transfer agent such as mercaptan. The toner containing a low molecular weight polypropylene (Japanese Patent Publication No. 3304/1977) and the toner obtained by using .alpha.,.beta.-unsaturated ethylene polymer having a broad molecular weight distribution, e.g. weight average molecular weight (Mw)/number average molecular weight (Mn)=3.5-40, as the toner resin (Japanese Patent Publication No. 6895/1980) are also known.
Besides the toner containing a vinyl polymer or its mixture which has at least one peak value in the respective region of 10.sup.3 -8.times.10.sup.4 and 10.sup.5 -2.times.10.sup.6 in molecular weight (Japanese Patent Laid-open No. 16144/1981, relevant to U.S. Pat. No. 4,499,168) is known.
The toner containing a low molecular weight polymer which has a number average molecular weight of 3,000-50,000 and Mw/Mn of less than 3.5, and an insoluble and infusible polymer (Japanese Patent Publication No. 86558/1983) has also been proposed recently.
The toner obtained from such polymer, however, is still unsatisfactory in charge stability, cannot provide a good image under high relative humidity in particular and results in severe problems in the actual use. In addition, the toner is apt to cause blocking under high relative humidity, and is difficult to employ in practical application.